Supercritical fluid extraction (SFE) and preparative (“Prep”) supercritical fluid chromatography (SFC) each require devices and processes for collecting liquid fractions from the terminus of the flow system. The major components of supercritical fluid (SF) mobile phases include highly pressurized gas such as carbon dioxide (CO2, or CO2) and liquid organic co-solvents, or modifiers. For collection, the mobile phase must be brought to near ambient pressure where the CO2 component is an expanded gas with up to 500 times the volume of the pressurized phase. A much smaller volume of residual liquid modifier containing the compounds of interest is entrained with the CO2, initially as an aerosol. As a result, the process of collecting the compounds of interest is more complicated than fraction collection in Preparative High Performance Liquid Chromatography (HPLC) where only liquids are used. The collection process in either Prep SFC or SFE involves several steps including: a) depressurization of the mobile phase; b) optional heating to restore heat of evaporation of the evaporating CO2 phase; c) partitioning of the resulting gas and liquid phases d) venting of the vapor and e) direction of the residual liquid phase to a suitable collection container.
Depressurization of the mobile phase is initiated when the mobile phase passes through a back pressure regulator (BPR), which is usually set to a control setpoint of 100 bar or greater to maintain ideal chromatographic/extraction conditions. Flow continues beyond the BPR in a very chaotic state of evaporating liquid CO2 and expanding vapor CO2. Such extreme turbulence within this flow region tends to aerosolize much of the residual organic liquid from the mobile phase. In addition, the evaporating CO2 causes considerable cooling of the residual liquid organic which allows it to retain a high concentration of dissolved CO2.
Flow is generally delivered via a transfer line from the BPR to a gas-liquid separator. The general role of the gas-liquid separator is to complete the expansion of the CO2 vapor to a designated pressure, and allow venting of the vapor while simultaneously collecting the residual liquid or redirecting it along a selected path to a collection container. In order to accomplish this task some method must be used to handle residual aerosols emerging from the flow transfer line. Since the degree of aerosolization depends upon both flow rate and composition of the mobile phase, the gas-liquid separator must be sized to handle appropriate ranges for both parameters.
Several techniques and devices for aerosol suppression in gas-liquid separators are known in the industry. FIG. 9 illustrates such a device 700 where the flow line 710 can be introduced into an open top-end of separator 720 in such a manner that the liquid aerosol 730 collides with the separator wall and coalesces to a film 750 which drains by gravity down the wall. Not shown is a multi-step heater assembly to condition the flow to separate the gas and liquid phases and dramatically suppress aerosol in the transfer tubing. The CO2 and any organic vapor 740 are allowed to vent out the top of the separator by pneumatic forces. It is known to hold the internal pressure of the separator at an elevated pressure in order to slow the velocity of the aerosol 730 escaping from narrow flow line 710. This has the effect of reducing the shear forces that might lift the coalesced liquid 760 upward on the wall or re-aerosolize the liquid film 750.
FIG. 10 illustrates another device 800 often used to slow gaseous flowstream (e.g. CO2) velocity by adjusting the internal diameter of the transfer line as it enters the separator. Flow line 810 can be introduced into an open top-end of separator 820 in such a manner that the liquid aerosol 830 collides with the separator wall and coalesces to a film 850 which drains by gravity down a wall. The CO2 and any organic vapor are allowed to vent 840 out the top of the separator by pneumatic forces. It is known to hold the internal pressure of the separator 800 at an elevated pressure in order to slow further the velocity of the aerosol 830. This has the effect of reducing the shear forces that might lift the coalesced liquid 860 upward on the wall or re-aerosolize the liquid film 850. The exit end 870 of transfer line 810 into separator 800 has an enlarged diameter that is some multiple of a normal diameter of tube 810. For example, by doubling the internal diameter (i.d.) of a round tube, the flow velocity is cut to 25% of its original velocity.
Regardless of the suppression type, most separators experience some level of fouling from liquid droplets 880 reaching poorly swept regions of the internal surface of the separator. This is problematic when the same separator must be used for different samples since it introduces a carryover or cross-contamination issue that destroys the integrity of subsequent collectable fractions into the separator. As a result, a manual or automated cleaning process is typically required to prepare the separator for the next fraction type. Generally this is accomplished by solvent rinsing of the reused parts of the collector. In one case use of a replaceable collection liner (e.g. test tube) removes the fouled surfaces with the collected fraction.
In either Prep SFC or SFE, mobile phase exiting the instrument is frequently enriched with dissolved compounds of interest that require collection. A flow segment containing either an individual or a group of such compounds and properly directed to a known container is called a fraction. Fractions may also be flow segments delivered out of the instrument in a given time window, whether they contain dissolved compounds or not. The apparatus that delivers different fractions to separate containers is called a fraction collector assembly. Gas-liquid separators are part of the fraction collector assembly in Prep SFC and SFE systems and are generally designed either for parallel or serial collection use. In a parallel configuration, an individual collector comprises a single gas/liquid separator in series with a single collection container. Frequently, the collection container itself is integral to the gas/liquid separator apparatus to form the collector. At other times the collection container is remote from the separator but dedicated to exclusively receiving its fractions. Individual collectors are then plumbed in multiple parallel collection paths depending on the maximum number of distinct fractions to be collected per separation. The individual path to a specific collector is generally determined by a valving arrangement which is also part of the fraction collector assembly. Parallel fraction collector assemblies typically collect a single chromatographically separated fraction in each collector for each separation. Multiple separations may pool identical fractions from separate separations into the same collector to enable collection of larger amounts of the compounds of interest. Regardless, each gas-liquid separator contacts only one type of fraction. Hence, there is no need for cleaning of the separator until all like fractions have been collected.
In a series gas-liquid separator arrangement, a single gas/liquid separator delivers the liquid portion of fractions to more than one collection container. As a result, more than one type of sample fraction flows through the separator for a given separation. Separators of this type are flow through and do not incorporate a single collection container as a permanent part of the collector. Instead, multiple collection containers are attachable to the separator exit port typically via a transfer line using a robotic or valving assembly to switch to each targeted collection container. This type of separator must be designed to clear each fraction rapidly and rinse itself, prior to the next fraction entering the separator. Also, the design must take into account not only the gas-liquid separation process, but the drain rate of the liquid to the separator's exit port that is typically located near the bottom of the separator. In addition, sufficient rinsing must be applied and cleared from the separator to minimize cross-contamination between fractions. Series gas-liquid separators can operate in a continuous manner or as an array of two or more separators that alternate to allow for both collecting and rinsing functions. They generally have much lower internal surface area than parallel separators in order to limit un-swept areas and minimize rinse volumes.
Limiting the drain time and avoiding excess surface area contact are critical in the design of such separators. Surfaces which coalesce the liquid fractions must be clean and allow free-flowing of the droplets to the collector exit port in as a little time as possible. Most prior collectors have the drawback that drain times vary considerably depending on the liquid composition of the mobile phase. Further, changing the chemical nature of the organic liquid phase for example from one of low viscosity to one of high viscosity can dramatically affect the drain time.
Given the constraints for series separators, it is not surprising to find their useful dynamic range significantly limited. For example, a commercial series separator currently available fixes the total flow rate of the mobile phase and maintains a constant level of a single type of organic liquid into the separator using a makeup pump. The inability to optimize flow rate and select various modifier solvents for the chromatographic separation represents a severe limitation of the system.
Additionally, both parallel and serial gas-liquid separators are subject internal pressurization. This places a constraint on the materials that can be used to construct the separator. For many high-pressure applications, metal cyclones are generally required. For lower pressure applications, however, it is often desirable to use transparent materials for the separator such as glass. Glass material places a practical upper limit on the amount of pressurization that can be safely achieved without breaking or exploding the separator. For example, as a safety factor a glass separator must generally be shielded in the event of over-pressurization.
What is clearly needed is a simplified, low pressure gas-liquid separator that can be used in either a parallel or serial collection configuration. The separator configurations should be self-cleaning without manual or significant manual intervention and promote a high degree of recovery of the liquid phase to maximize solute-of-interest recovery.